Prevention of Porosity Formation and Other Effects of Gaseous Elements in Iron Castings

摘要

Iron foundries have observed porosity primarily as interdendritic porosity in large freezing range alloys such as Ni-Hard I and hypoeutectic high Cr alloys or pinholes and fissure defects in gray and ductile irons. For most iron foundries, porosity problems occur sporadically, but even occasional outbreaks can be costly since even a very small amount of porosity can significantly reduce the mechanical properties of the castings. As a result when porosity is detected, the castings are scrapped and remelted, or when the porosity is undetected, defective parts are shipped to the consumer. Neither case is desirable. This project was designed to examine various factors contributing to the porosity formation in iron castings. Factors such as solubility of gases in liquid and solid iron alloys, surface tension of liquid iron alloys, and permeability of dendritic structures were investigated in terms of their effect on the porosity formation. A method was developed to predict how much nitrogen the molten alloy picks up from air after a given amount of holding time for a given melting practice. It was shown that small batches of iron melts in an induction furnace can end up with very high concentration of nitrogen (near solubility limit). Surface tension of liquid iron alloys was measured as a function of temperature. Effect of minor additions of S, Ti, and Al on the surface tension of liquid iron alloys was investigated. Up to 18% change in surface tension was detected by minor element additions. This translates to the same amount of change in gas pressure required in a bubble of a given size to keep the bubble stable. A new method was developed to measure the permeability of dendritic structures in situ. The innovative aspect of these experiments, with respect to previous interdendritic permeability measurements, was the fact that the dendritic structure was allowed to form in situ and was not cooled and re-heated for permeability tests. A permeability model was developed and tested using the results of the permeability experiments. The permeability model for flow parallel to the columnar dendrites predicted the experimental permeability results closely when the liquid volume fraction data from equilibrium calculations were used. The permeability gradient model was constructed in order to test the impact of interdendritic channel constriction on the flow of liquid through the mushy zone of a casting. The model examines two different regimes: (i) Dendritic solidification regime where the permeability is dominated by changes in liquid volume fraction and dendrite arm spacing, and (ii) Eutectic solidification regime where the permeability is dominated by changes in viscosity of eutectic mixture. It is assumed that the eutectic mixture behaves like a slurry whose viscosity increases with increasing solid fraction. It is envisioned that this model can be developed into a tool that can be very useful for metal casters.